To make the rhubarb syrup, combine rhubarb, sugar and water in a large saucepan. Bring to a boil and then reduce heat to a simmer and cook until rhubarb breaks down, about 10 minutes. Take the syrup off the heat and stir in the vanilla, if using.,______,To make the cocktail, fill a shaker half full of ice.,Add rhubarb syrup, vodka, lemon juice and bitters. Shake vigorously for 15-20 seconds.,Pour into a small champagne glass (not a flute). Top each cocktail with Dry Sparkling Fuji Apple. Garnish with lemon zest.